1. Field of the Invention
This invention relates to a synthetic paper made of a stretched polyolefin resin film which is excellent particularly in offset printability and inkjet ink fixing properties and an inkjet recording paper with the use of this synthetic paper.
There have been known synthetic papers made of stretched films comprising as the support crystalline polyolefin resin compositions such as polypropylene and high-density polyethylene containing calcium carbonate powders of 0.8 to 4 .mu.m in average particle size (U.S. Pat. No. 4,341,880, No. 4,340,639 and No. 4,191,719). These synthetic papers have been marketed under the trademarks of "YUPO FPG", "YUPO KPG", "YUPO SGC", etc. from Oji Yuka Koseishi Co., Ltd. and "Polyart II" from BXL in UK.
Calcium carbonate powders employed in these synthetic papers involve: (1) dry-milled heavy calcium carbonate particles having an average particle size of from 1 to 10 .mu.m; (2) those having a desired particle size (0.5 to 1 .mu.m) by sieving or classing the above-mentioned dry-milled particles; (3) synthesized colloidal calcium carbonate particles having a particle size of from 0.03 to 0.2 .mu.m; (4) dry-milled heavy calcium carbonate particles having been surface-treated with metal salts of fatty acids; and (5) calcium carbonate particles carrying dispersant adhered thereto which have been obtained by dispersing calcium carbonate in an aqueous medium with the use of anionic polymer dispersants, wet-milling and then drying.
Since the dry-milled heavy calcium carbonate particles have a large average particle size of 1 .mu.m or above, voids are formed around these particles in the stretched film and, in its turn, cracks appear on the surface of the film. In the step of offset printing, inks filter into these cracks, which causes loss of gloss. Accordingly, these calcium carbonate particles are usable not in high-quality printing such as posters with excellent gloss but exclusively in printing papers for common uses such as greeting cards and books.
When blended with a crystalline polyolefin, the heavy calcium carbonate particles having been surface-treated with metal salts of fatty acids during dry-milling improve the dispersibility but exert no effect of preventing loss of gloss. The calcium carbonate particles obtained by dispersing calcium carbonate particles in an aqueous medium with the use of an anionic dispersant, milling and then drying are hardly usable in synthetic papers, because of the difficulties in milling them into primary particles and the serious secondary aggregation.
On the other hand, light calcium carbonate particles cannot be well dispersed in a crystalline polyolefin resin and frequently undergo aggregation due to the small average particle size thereof (0.2 .mu.m or less). After stretching, these aggregates appear as large projections on the surface of the synthetic papers. Then these projections are separated off from the paper surface and thus cause an image defect called white spots, which disturbs printing with high sharpness (JP-B-6-55549, JP-B-5-51900; the term "JP-B" as used herein means an "examined Japanese patent publication").
Under these circumstances, the present invention aims at providing a paper for offset printing allowing quick drying of inks and suffering from no loss of gloss, a synthetic paper which is suitable for gravure printing, etc., and an inkjet recording paper with the use of this synthetic paper.